Aerosol type sprayers are used throughout the world for dispensing a wide range of products, for example hair lacquer, furniture polish, cleaners, paint, insect killers and medicaments. Until recently, chlorofluorocarbons (CFC's) were the most common of the propellant gases used in aerosols because they are inert, miscible with a wide range of products, are easily liquefied under low pressures, give a substantially constant product flow-rate, and can produce sprays of droplets having mean diameters in the range of 3 to over 100 micrometers. However, in the 1970's it was confirmed that CFC's were probably responsible for depleting the Earth's protective ozone layer, and in 1987, most countries signed the Montreal Protocol to phase out the use of CFC's. Alternative propellants were then introduced--for example liquefied hydrocarbon gases such as butane, and carbon dioxide, which is dissolved in the product, --but these are flammable or otherwise harmful to the environment, or react with the product, and these propellant gases are gradually being phased out. There has been much development of aerosols powered by compressed gas (e.g. nitrogen, air), and manually operated pump atomizers, and for the majority of applications the performance of such sprayers is adequate.
The main drawback of these non-CFC sprayers is that the smallest sized droplet that can be produced is about 40 micrometers diameter, and despite considerable development of so-called mechanical breakup nozzles, the use of high pressure (circa 15 bars) pumps, and low viscosity/surface tension product formulations, 40 micrometers appears to be the lower limit achievable with prior art methods and devices.
There are aerosol generators used for research and hospital applications, such as ultrasonic nebulisers and spinning disc generators, but neither is suitable for portable, convenient atomizers.
It is also possible to force liquid at high pressure through a very small hole (5-10 micrometers diameter) to produce droplets of about 5 micrometers diameter, but these methods are unsuitable or uneconomic for large scale manufacture, mainly because of the difficulty in making very small holes in a suitable material, and, to prevent blockage of the hole, the need for exceptional cleanliness in the manufacture of the parts, together with ultrafiltration of the fluid to be sprayed.
For veterinary and some human vaccination applications, high pressure (125-500 bars) spring or gas operated pumps (so-called needle-less injectors) are in common use to inject a jet of drug through the skin ("intra-dermal injection") without the use of needles, and attachments are available to convert the jet to a spray for administering drugs to the nasal passages of large animals such as swine. However, the smallest droplet size obtainable is in the order of 40 micrometers, and the range of applications for these injectors is limited.
Compressed air atomizers such as air brushes and commercial paint sprayers consume large quantities of air, and to obtain droplets of 5 micrometers with water for example, a gas to liquid ratio of over 30,000:1 is required, which is impractical for convenient, portable sprayers.
Nevertheless, there are some applications that rely on a smaller droplet size for maximum efficacy: space sprays such as flying insect killers should contain droplets ideally in the range of 20-30 micrometers diameter to ensure a long flotation time in the air, and for metered dose inhalers (MDI's) used for treating certain respiratory disorders it is essential that the aerodynamic particle size should be less than 15 micrometers, preferably less than 5 micrometers, so that the droplets are able to penetrate and deposit in the tracheobronchial and alveolar regions of the lung. For a spray composed of droplets with a range of sizes, more than 5% by weight of the droplets should have an aerodynamic size less than 6.4 micrometers, preferably more than 20 by weight of the particles have an aerodynamic size less than 6.4 micrometers.
Inhalers may also be designed to deliver drugs to the alveolar sacs of the lung to provide a route for adsorption into the blood stream of drugs that are poorly adsorbed from the alimentary tract. To reach the alveoli it is essential that the aerodynamic size of the particles is less than 10 micrometers, preferably 0.5-5 micrometers.
Many of the drugs used in the treatment of respiratory disorders are insoluble in vehicles such as water and are dispensed as suspensions. The drugs are produced in a respirable size of 1-5 micrometers. Particles of this size tend to block the very small holes (5-10 micrometers) used by known devices to generate droplets of about 5 micrometers diameter.